The present invention relates to a method for braking a traction sheave elevator, to a traction sheave elevator and to the use of an emergency power supply.
The machine of a traction sheave elevator consists of a traction sheave with the elevator hoisting ropes fitted to its grooves and an electric motor driving the traction sheave either directly or via a gear. The machine is provided with a brake, which applies a braking force to the traction sheave either directly or e.g. via the shaft. The operating brake of the elevator works under positive control such that the brake is always braking when it is not specifically caused by the control system not to brake. In a typical operating brake construction, the brake is closed by the force of a spring or an equivalent element, and a controllable actuator counteracting the closing element releases the brake and keeps it released. When braking is applied to the traction sheave, the braking effect is transmitted further to the elevator ropes by the action of the frictional grip and other gripping forces applied by the traction sheave to the ropes. In an emergency braking situation where the elevator is stopped as quickly as possible, a greater gripping force is likely to be needed than when the elevator is being accelerated and decelerated during normal operation.
To improve the grip between the ropes and the traction sheave, the grooves on the traction sheave of especially fast elevators and elevators with a large hoisting height are sometimes heavily undercut. The grip can also be improved by increasing the rope angle. Solutions increasing the rope angle are e.g. ESW (extended single wrap) and double-wrap suspension arrangements, in which crosswise roping or a secondary rope pulley is used to achieve a contact angle larger than 180 degrees between the ropes and the traction sheave. In conventional single-wrap suspension (CSW), the contact angle between the ropes and the traction sheave is 180xc2x0 or somewhat less if the distance between the ropes is increased using a diverting pulley. Thus, both undercut rope grooves and increasing the undercut as well as increasing the contact angle improve the grip.
For normal operation, in most elevators, including high-rise and fast elevators, conventional single-wrap suspension with the hoisting ropes only passing over the traction sheave and a very moderate undercut in the grooves of the traction sheave would be sufficient to guarantee a non-slip grip between the traction sheave and the ropes with all elevator load alternatives. However, to provide for emergency braking, the system has to be designed so as to ensure a better grip. Improving the grip, however, leads to drawbacks that increase the costs of the elevator, especially the costs arising during operation. An undercut promotes wear of the rope and the rope groove; the larger the undercut, the faster the wear. Similarly, in ESW and double-wrap suspension, rope bends following each other at short distances increase rope wear. In ESW and double-wrap suspension, rope skew is another factor increasing rope wear. Double-wrap suspension produces a particularly hard strain on the bearings of the traction sheave and the secondary rope pulley.
On the other hand, a point to remember is that emergency braking must not be too effective. If the braking is too effective, the rapid deceleration of the elevator car may involve a danger to the passengers. A deceleration rate exceeding gravitational acceleration during upward travel of the elevator is sufficient for the passengers to loose contact with the floor of the elevator car. Depending on the initial deceleration rate, this will cause the passengers to be hurled against the ceiling of the elevator car or at least to tumble.
The object of the invention is to remedy the above-mentioned defects and at the same time to extend the use of conventional elevator suspension based on an advantageous fundamental solution to elevators designed for higher speeds or a greater hoisting height. Another object of the invention is to disclose an easy method for utilizing a brake not comprised in the drive machinery in a situation where passengers are to be freed from an elevator that has stopped due to a power failure.
The invention makes it possible to extend the safe field of application of CSW-type elevators to elevators designed for higher speeds or a greater hoisting height without having to compromise on the useful life of the ropes or the traction sheave as a consequence of a significantly improved grip between the ropes and the traction sheave. Using a simple arrangement, the invention also leads to an improvement in the operating characteristics of fast and high-rise elevators. Safe extension of the field of application is achieved by increasing the braking force applied during emergency braking and at the same time taking care that the deceleration of the elevator car is not increased excessively. In high-rise elevators, which are among the fastest elevators, the mass of the car typically equals two to two-and-a-half times the nominal load while the mass of the counterweight typically equals the mass of the car plus half the nominal load. Additional masses to be accelerated in the elevator include e.g. the mass of the ropes. When, according to the basic idea of the invention, the decelerating force generated by a braking device not comprised in the drive machine is kept at a clearly lower level than the weight of the nominal load of the elevator, harmful deceleration rates during emergency braking of the elevator are avoided.
As the braking device not comprised in the drive machine but placed at a large distance from the elevator machine room has to be released using an emergency power supply from the machine room or the release of the brake has to be effected in some other way from a distance, the emergency power supply or other emergency device used can be a device with moderate ratings, because the braking device is of a moderate size regarding its braking force and the energy required for its release and therefore also its efficiency.
Using the solution of the invention, a longer useful life of the ropes and traction sheave are achieved. The drive machine can be implemented using a solution involving no large internal stresses, thus reducing e.g. the load on the bearings. The service life of the ropes, traction sheave and bearings may even be increased to several times their usual durability. On the whole, simpler solutions regarding the machine and rope suspension can be achieved. As CSW suspension does not require any voluminous diverting pulley arrangements in the machine room, even a very large elevator will only need a moderate machine room floor area. No heavy supporting structures for diverting pulley arrangements are needed. The moderate size and weight of the machine achieved by the invention allow easier machine room lay-out and installation work. High-performance machines are often part of an elevator group of several elevators, and in this situation the advantage regarding space utilization provided by easy placeability is accentuated. The brake not comprised in the drive machine as provided by the invention can be used safely and without any major special measures in a situation where passengers are to be freed from an elevator that has stopped due to a power failure.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.